Corn (Zea mays L.) is grown widely in the United States and has been the subject of breeding programs and human manipulation for centuries. Corn is a monoecious plant, meaning one plant has both male and female flowers. The male flower is located in the tassel at the top of the corn plant and produces pollen. The female flower is located in the ear shoot on the stalk of the corn plant and receives pollen from the male flower. Thus, corn is capable of self-pollination (pollen from the male flower is received by the female flower of the same plant) or is capable of cross-pollination (pollen from the male flower is received by the female flower of a different plant). Sib-pollination occurs when the female flower of one corn plant receives pollen from the male flower of another corn plant of the same line.
Inbred lines are developed by allowing or directing a plant to self-pollinate or sib-pollinate for a number of generations. After each generation, plants with a desired trait or a combination of desired traits are selected and allowed to self-pollinate or sib-pollinate again. The process is repeated until the population is homozygous and true breeding for that trait or combination of traits. Usually, inbred corn lines are developed so that they can then be crossed with a different inbred corn line to create new hybrid corn varieties. Due to a phenomenon known as hybrid vigor, inbred corn lines are typically not as vigorous or high yielding as hybrid corn varieties. Thus, the plants of the hybrid corn varieties typically exhibit better agronomic characteristics than the plants of the parent inbred corn lines.